media: staging: ipu3-imgu: Document pages field

The pages field in struct imgu_css_map was missing. Document it.
Reported-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

drivers/staging/media/ipu3/ipu3-css-pool.h

@@ -15,6 +15,7 @@ struct imgu_device;
  * @size:		size of the buffer in bytes.
  * @vaddr:		kernel virtual address.
  * @daddr:		iova dma address to access IPU3.
+ * @pages:		pages mapped to this buffer
  */
 struct imgu_css_map {
 	size_t size;